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High-resolution photoemission and first-principles density-functional slab calculations were used to study the adsorption of gold on a TiC(001) surface. A positive shift in the binding energy of the C 1s core level is observed after the deposition of Au on the metal carbide surface. The results of the density-functional calculations corroborate the formation of Au–C bonds. In general, the bond between Au and the TiC(001) surface exhibits very little ionic character, but there is a substantial polarization of electrons around Au that affects its chemical properties. [ABSTRACT FROM AUTHOR]

A systematic study of the bulk and surface geometrical and electronic properties of a series of transition-metal carbides (TMC with TM=Ti, V, Zr, Nb, Mo, Hf, Ta, and W) by first-principles methods is presented. It is shown that in these materials the chemical bonding is strongly covalent, the cohesive energies being directly related to the bonding-antibonding gap although the shift of the center of the C(2s) band related peak in the density of states with respect to diamond indicates that some metal to carbon charge transfer does also take place. The (001) face of these metal carbides exhibits a noticeable surface rumpling which grows along the series. It is shown that neglecting surface relaxation results in very large errors on the surface energy and work function. The surface formation induces a significant shift of electronic energy levels with respect to the corresponding values in the bulk. The extent and nature of the shift can be understood from simple bonding-antibonding arguments and is enhanced by the structural rippling of this surface. [ABSTRACT FROM AUTHOR]

The oxygen reduction reaction (ORR) stands as a pivotal process in electrochemistry, finding applications in various energy conversion technologies such as fuel cells, metal-air batteries, and chlor-alkali electrolyzers. Hereby, a comprehensive density functional theory (DFT) investigation is presented into the proposed conventional and unconventional ORR mechanisms using single-atom catalysts (SACs) supported on nitrogen-doped graphene (NG) as model systems. Several reaction intermediates have been identified that appear to be more stable than the ones postulated in the conventional mechanism, which follows the *OOH, *O, and *OH intermediates. This finding particularly holds for adsorbed *O 2 , which can have different adsorption geometries, ranging from η 1 Ο 2 or η 2 Ο 2 superoxo complexes as well as sin and anti complexes, with the two O-related ligands binding on the same or opposite sides, respectively. In the case of M@NG (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pt), the ORR follows these unconventional *O 2 intermediates, whereas for Cr@NG and Cu@NG classical and unconventional *O 2 intermediates compete. We approximate the electrocatalytic activity using the concept of the thermodynamic overpotential and demonstrate that the conventional mechanism gives rise to a smaller overpotential compared to mechanisms following unconventional intermediates during the four proton-coupled electron transfer steps. Our trend study indicates that transition metals with fewer d electrons reveal smaller electrocatalytic activity due to a larger thermodynamic overpotential. Among the investigated SAC systems, Co emerges as a promising candidate, with thermodynamic overpotential and limiting potential values of 0.38 and 0.85 V vs the standard hydrogen electrode, respectively, with the conventional mechanism being favored, and with Cu appearing as the second-best candidate.

The interaction between molybdenum carbide (MoC y ) nanoparticles and both flat and curved graphene surfaces, serving as models for carbon nanotubes, was investigated by means of density functional theory. A variety of MoC y nanoparticles with different sizes and stoichiometries have been used to explore different adsorption sites and modes across models with different curvature degrees. On flat graphene, off-stoichiometric MoC y featuring more low-coordinated Mo atoms exhibits stronger interaction and increased electron transfers from the carbide to the carbon substrate. This preferentially occurs through support C and Mo atoms leading to the formation of additional Mo-C bonds. Notably, the MoC y adsorption strength increases on concave surfaces and decreases on convex surfaces, showing a strong linear correlation with the surface curvature. This curvature-dependent behavior alters the charge state of the nanoparticles, making them more/less positively charged in concave/convex regions. The present results demonstrate that the interaction strength can be effectively tuned by manipulating the carbide stoichiometry, the substrate curvature, and the local concave/convex environments, providing valuable guidelines for the rational design of MoC y /C-based catalysts.

The separation of oxygen (O 2 ) and nitrogen (N 2 ) from air is a process of utmost importance nowadays, as both species are vital for numerous fundamental processes essential for our development. Membranes designed for their selective molecule separation have become the materials of choice for researchers, primarily due to their ease of use. The present study proposes grazynes, 2D carbon-based materials consisting of sp and sp 2 C atoms, as suitable membranes for separating O 2 and N 2 from air. By combining static density functional theory (DFT) calculations with molecular dynamics (MD) simulations, we address this issue through a comprehensive examination of the thermodynamic, kinetic, and dynamic aspects of the molecular diffusions across the nano-engineered pores of grazynes. The studied grazyne structures have demonstrated the ability to physisorb both O 2 and N 2 , preventing material saturation, with diffusion rates exceeding 1 s -1 across a temperature range of 100-500 K. Moreover, they exhibit a selectivity of ca. 2 towards O 2 at 300 K. Indeed, MD simulations with equimolar mixtures of O 2 :N 2 indicated a selectivity towards O 2 in both grazynes with ca . twice as many O 2 filtered molecules in the [1],[2]{2}-grazyne and with O 2 representing ca. 88% of the filtered gas in the [1],[2]{(0,0),2}-grazyne. [1],[2]{2}-grazyne shows higher permeability for both molecules compared to the other grazyne, with O₂ demonstrating particularly enhanced diffusion capacity across both membranes. Further MD simulations incorporating CO 2 and Ar confirm O 2 enrichment, particularly with [1],[2]{(0,0),2}-grazyne, which increased the presence of O 2 in the filtered mixture by 26% with no evidence of CO 2 molecules.

First-principles density functional theory calculations are carried out on the (TiO 2 ) 5 cluster supported on the Ti 2 CT x (0001) surface with different chemical terminations, i.e. , -H, -O, and -OH, to study the interaction and understand the Ti 2 CT x functionalization effect on the formation of (TiO 2 ) 5 /Ti 2 CT x composites. Results show an exothermic interaction for all cases, whose strength is driven by the surface termination, promoting weaker bonds when the MXene is functionalized with H atoms. For Ti 2 CH 2 and Ti 2 C(OH) 2 MXenes, the interaction is accompanied by a charge transfer towards the titania cluster. All adsorptions are accompanied by a significant structural deformation of the titania nanocluster. The analysis of the density of states of (TiO 2 ) 5 /Ti 2 CH 2 and (TiO 2 ) 5 /Ti 2 C(OH) 2 composites shows a clear almost metallic character with titania-related states close to the Fermi level. However, for (TiO 2 ) 5 /Ti 2 CO 2 , the band positions are similar to those of a Type-I heterojunction. Overall, the MXene surface termination influence on the TiO 2 /MXene interaction is unveiled, providing more stable composite formations when the MXene surface is functionalized with -H and -OH groups, where the adsorption process is accompanied by significant charge transfer., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Hydrogen adatoms are involved in many reactions catalyzed by Transition Metal (TM) surfaces, such as the Haber-Bosch process or the reverse water gas shift reaction, key to our modern society. Any rational improvement on such a catalyst requires an atomistic knowledge of the metal↔hydrogen interaction, only attainable from first-principles calculations on suited, realistic models. The present thorough density functional theory study evaluates such H interaction at a low coverage on most stable surfaces of bcc , fcc , and hcp TMs. These are (001), (011), and (111) for bcc and fcc TMs and (0001), (101̅0), and (112̅0) for hcp , covering 27 TMs and 81 different TM surfaces in total. In general terms, the results validate, while expanding, previous assessments, revealing that TM surfaces can be divided into two main groups, one in the majority where H 2 would be thermodynamically driven to dissociate into H adatoms, located at heights of ∼0.5 or ∼1.0 Å, and another for late TMs, generally with a d 10 electronic configuration, where H 2 adsorption with no dissociation would be preferred. No trends in H adsorption energies are found down the groups, but yes along the d series, with a best linear adjustment found for the d -band center descriptor, especially suited for close-packed fcc and hcp TMs surfaces, with a mean absolute error of 0.15 eV. Gibbs free adsorption energies reveal a theoretical volcano plot where fcc TMs are best suited, but with peak Pt performance displaced due to dispersive force inclusion in the method. Still, the volcano plot with respect to the experimental logarithm of the exchanged current density polycrystalline data is far from being valid for a quantitative assessment, although useful for a qualitative screening and to confirm the trends computationally observed., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

First-principles calculations based on density functional theory are performed to investigate the formation of titania/MXene composites taking (TiO 2 ) 5 /M 2 C (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) as cases of study. The present systematic analysis confirms a favorable, high exothermic interaction, which promotes important structural reconstructions of the (TiO 2 ) 5 cluster along with charge transfer from the MXene to titania. MXenes composed of d 3 transition metals promote the strongest interaction, deformation energy, and charge transfer, followed by d 4 and d 5 M 2 C MXenes. We provide evidence that the formation of these (TiO 2 ) 5 /M 2 C composites is governed by charge transfer, leading to scaling relationships. By using the electronegativity of the metal composing MXene and the MXene d-band center, we also establish linear correlations that can be used to predict the interaction strength of (TiO 2 ) 5 /M 2 C composites just from the knowledge of the MXene composition. It is likely that the present trends hold for other TiO 2 /MXene composites.

The performance of B-containing Ti 3 C 2 MXenes as catalysts for the nitrogen reduction reaction (NRR) is scrutinized using density functional theory methods on realistic models and accounting for working conditions. The present models include substituted and adsorbed boron along with various mixed surface terminations, primarily comprising -O and -OH groups, while considering the competitive hydrogen evolution reaction (HER) as well. The results highlight that substituted and low-coordinate adsorbed boron atoms exhibit a very high N 2 adsorption capability. For NRR, adsorbed B atoms yield lower limiting potentials, especially for surfaces with mixed -O/-OH surface groups, where the latter participate in the reaction lowering the hydrogenation reaction energy costs. The NRR does also benefit of having B adsorbed on the surface which on moderate -OH terminated model displays the lowest limiting potential of -0.83 V, competitive to reference Ru and to HER. The insights derived from this comprehensive study provide guidance in formulating effective MXene-based electrocatalysts for NRR., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

The separation of carbon dioxide (CO 2 ) from nitrogen (N 2 ) is at the core of any global warming remediation technology aimed at reducing the CO 2 content in the atmosphere. Chemical membranes designed to differentially permeate both molecules have become quite appealing due to their simple use, although many membrane-based separations stand out as a promising solution for CO 2 separation. These are environmentally friendly, with high active surface areas, compact design, easy to maintain and cost-effective, although the field is still growing due to the difficulties in the CO 2 /N 2 separation. The present study poses grazynes, two-dimensional C-based materials with sp and sp 2 C atoms, aligned along stripes, as suited membranes for the CO 2 /N 2 separation. The combination of density functional theory (DFT) and molecular dynamics (MD) simulations allow tackling the energetics, kinetics, and dynamics of the membrane effectiveness of grazynes with engineered pores for such a separation in a holistic fashion. The explored grazynes are capable of physisorbing CO 2 and N 2 , thus avoiding material poisoning by molecular decoration, while the diffusion of CO 2 through the pores is found to be rapid, yet easier than that of N 2 , in the rate order of the s -1 in the 100-500 K temperature range. In particular, low-temperature CO 2 separation even for CO 2 contents below 0.5 % are found for [1],[2]{2}-grazyne when controlling the membrane exposure contact to the gas mixture, paving the way for exploring and using grazynes for air CO 2 remediation., (© 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

Kinetic studies are vital for gathering mechanistic insights into heterogeneously catalyzed hydrogenation of unsaturated organic compounds (olefins), where the Horiuti-Polanyi mechanism is ubiquitous on metal catalysts. While this mechanism envisions nonpairwise H 2 addition due to the rapid scrambling of surface hydride (H*) species, a pairwise H 2 addition is experimentally encountered, rationalized here based on density functional theory (DFT) simulations for the ethene (C 2 H 4 ) hydrogenation catalyzed by two-dimensional (2D) MXene Mo 2 C(0001) surface and compared to Rh(111) surface. Results show that ethyl (C 2 H 5 *) hydrogenation is the rate-determining step (RDS) on Mo 2 C(0001), yet C 2 H 5 * formation is the RDS on Rh(111), which features a higher reaction rate and contribution from pairwise H 2 addition compared to 2D-Mo 2 C(0001). This qualitatively agrees with the experimental results for propene hydrogenation with parahydrogen over 2D-Mo 2 C 1- x MXene and Rh/TiO 2 . However, DFT results imply that pairwise selectivity should be negligible owing to the facile H* diffusion on both surfaces, not affected by H* nor C 2 H 4 * coverages. DFT results also rule out the Eley-Rideal mechanism appreciably contributing to pairwise addition. The measurable contribution of the pairwise hydrogenation pathway operating concurrently with the dominant nonpairwise one is proposed to be due to the dynamic site blocking at higher adsorbate coverages or another mechanism that would drastically limit the diffusion of H* adatoms., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

The synthesis and properties of stoichiometric, reduced, and Co-doped In 2 O 3 are described in the light of several experimental techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet (UV)-visible spectroscopy, porosimetry, and density functional theory (DFT) methods on appropriate models. DFT-based calculations provide an accurate prediction of the atomic and electronic structure of these systems. The computed lattice parameter is linearly correlated with the experimental result in the Co concentration ranging from 1.0 to 5.0%. For higher Co concentrations, the theoretical-experimental analysis of the results indicates that the dopant is likely to be preferentially present at surface sites. The analysis of the electronic structure supports the experimental assignment of Co 2+ for the doped material. Experiments and theory find that the presence of Co has a limited effect on the material band gap.

The search for cheap and active materials for the capture and activation of CO 2 has led to many efforts aimed at developing new catalysts. In this context, earth-abundant transition metal carbides (TMCs) have emerged as promising candidates, garnering increased attention in recent decades due to their exceptional refractory properties and resistance to sintering, coking, and sulfur poisoning. In this work, we assess the use of Group 5 TMCs (VC, NbC, and TaC) as potential materials for carbon capture and sequestration/utilization technologies by combining experimental characterization techniques, first-principles-based multiscale modeling, vibrational analysis, and catalytic experiments. Our findings reveal that the stoichiometric phase of VC exhibits weak interactions with CO 2 , displaying an inability to adsorb or dissociate it. However, VC often exhibits the presence of surface carbon vacancies, leading to significant activation of CO 2 at room temperature and facilitating its catalytic hydrogenation. In contrast, stoichiometric NbC and TaC phases exhibit stronger interactions with CO 2 , capable of adsorbing and even breaking of CO 2 at low temperatures, particularly notable in the case of TaC. Nevertheless, NbC and TaC demonstrate poor catalytic performance for CO 2 hydrogenation. This work suggests Group 5 TMCs as potential materials for CO 2 abatement, emphasizes the importance of surface vacancies in enhancing catalytic activity and adsorption capability, and provides a reference for using the infrared spectra as a unique identifier to detect oxy-carbide phases or surface C vacancies within Group 5 TMCs.

Context: The present work provides a systematic theoretical analysis of the nature of the chemical bond in Al 2 O 3 , Ga 2 O 3 , and In 2 O 3 group 13 cubic crystal structure metal oxides. The influence of the functional in the resulting band gap is assessed. The topological analysis of the electron density provides unambiguous information about the degree of ionicity along the group which is linearly correlated with the band gap values and with the cost of forming a single oxygen vacancy. Overall, this study offers a comprehensive insight into the electronic structure of metal oxides and their interrelations. This will help researchers to harness information effectively, boosting the development of novel metal oxide catalysts or innovative methodologies for their preparation., Methods: Periodic density functional theory was used to predict the atomic structure of the materials of interest. Structure optimization was carried out using the PBE functional, using a plane wave basis set and the PAW representation of the atomic cores, using the VASP code. Next, the electronic properties were computed by carrying out single point calculations employing PBE, PBE + U functionals using VASP and also with PBE and the hybrid HSE06 functionals using the FHI-AIMS software. For the hybrid HSE06, the impact of the screening parameter, ω, and mixing parameter, α, on the calculated band gap has also been assessed., (© 2024. The Author(s).)

First-principles calculations on titania clusters (TiO 2 ) n (n=5 and 10) supported on the pristine Ti 2 C (0001) surface were carried out to understand the properties of semiconductor/MXene composites with implications in (photo)-catalysis. The reported results reveal a high exothermic interaction accompanied by a substantial charge transfer with a concomitant, notorious, deformation of the titania nanoclusters. The analysis of the density of states analysis of the composite systems evidences a metallic character with titania related states crossing the Fermi level. The picture of the chemical bonds is completed by the analysis of X-Ray Photoelectron Spectra (XPS) features, evidencing clear shifts of the C(1s) and O(1s) related peaks relative to the isolated systems that have a quite complex origin. This detailed analysis provides insights to experimentalists interested in the design and synthesis of these systems with possible applications in catalysis., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Density functional theory would be exact when the exact exchange-correlation ( xc ) functional would be known, but since it is regretfully not known, dozens of xc functionals have been developed in the past decades, with some of them better suited for describing certain systems and/or properties. For transition metals (TMs), recent systematic studies assessing bulk properties─shortest interatomic bond distance, δ, cohesive energy, E coh , and bulk modulus, B 0 ─and surface features─surface energy, γ, work function, ϕ, and interlayer distances, δ ij functionals for the TMs bulk and surfaces description, such as Perdew-Burke-Ernzerhof (PBE) or Vega-Viñes (VV). Still, some basic local density approximation xc functionals are, overall, better suited than other types of xc functionals for the TMs bulk and surfaces description, such as Perdew-Burke-Ernzerhof (PBE) or Vega-Viñes (VV). Still, some basic local density approximation xc functionals were not assessed, such as the Hedin-Lundqvist (HL) and Perdew-Zunger (PZ), or GGAs such as the revised Perdew-Burke-Ernzerhof (revPBE) or the Armiento-Mattsson (AM05). Here, we expand the analysis by not only including them but also the recent meta-GGA strongly constrained appropriately normed (SCAN) xc functional, characterized by fulfilling all 17 mathematical conditions an xc must comply, plus the Bayesian error estimation functional (BEEF) xc , a functional parametrized over a large and diverse set of experimental results using machine learning. The present results reveal that none of the xc studied excel neither PBE nor VV, yet AM05 and SCAN performance is quite acceptable, while BEEF xc probably needs more shells of parametrization to reach competitive accuracy levels.

A systematic computational study is presented aimed at accurately describing the electronic ground state nature and properties of M 2 C (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) MXenes. Electronic band structure calculations in the framework of density functional theory (DFT), carried out with different types of basis sets and employing the generalized gradient approach (GGA) and hybrid functionals, provide strong evidence that Ti 2 C, Zr 2 C, Hf 2 C, and Cr 2 C MXenes exhibit an open-shell conducting ground state with localized spins on the metal atoms, while V 2 C, Nb 2 C, Mo 2 C, Ta 2 C, and W 2 C MXenes exhibit a diamagnetic conducting ground state. For Ti 2 C, Zr 2 C, Hf 2 C, and Cr 2 C, the analysis of the low-lying spin polarized solutions with different spin orderings indicates that their ground states are antiferromagnetic (AFM), consisting of two ferromagnetic (FM) metal layers coupled antiferromagnetically. For the diamagnetic MXenes, the converged spin polarized solutions are significantly less stable than the closed shell solution except for the case of V 2 C and Mo 2 C where those excited open shell solutions can be thermally accessible (less than 300 meV per formula unit). The analysis of charge and spin density distributions of the ground state of the MXenes reveals that, in all cases, the metal atoms have a net charge close to +1 e and C atoms close to -2 e . In the case of diamagnetic MXenes, the electronic structure of V 2 C, Nb 2 C, and Ta 2 C is consistent with metal atoms exhibiting a closed-shell s 2 d 2 configuration whereas for Mo 2 C, and W 2 C is consistent with a low-spin s 1 d 4 configuration although the FM solution is close in energy for V 2 C and Mo 2 C suggesting that they may play a role in their chemistry at high temperature. For the open shell MXenes, the spin density primarily located at the metal atoms showing one unpaired electron per Ti + , Zr + , and Hf + magnetic center, consistent with s 2 d 1 configuration of the metal atom, and of ∼3.5 unpaired electrons per Cr + magnetic center interpreted as a mixture of s 2 d 3 and high-spin s 1 d 4 configuration. Finally, the analysis of the density of states reveals the metallic character of all these bare MXenes, irrespective of the nature of the ground state, with significant covalent contributions for Mo 2 C and W 2 C.

The magnetic nature of Ti 2 C, Ti 3 C 2 , and Ti 4 C 3 MXenes is determined from periodic calculations within density functional theory and using the generalized gradient approximation based PBE functional, the PBE0 and HSE06 hybrids, and the on-site Hubbard corrected PBE+ U one, in all cases using a very tight numerical setup. The results show that all functionals consistently predict a magnetic ground state for all MXenes, with spin densities mainly located at the Ti surface atoms. The analysis of solutions corresponding to different spin orderings consistently show that all functionals predict an antiferromagnetic conducting ground state with the two ferromagnetic outer (surface) Ti layers being antiferromagnetically coupled. A physically meaningful spin model is proposed, consistent with the analysis of the chemical bond, with closed shell, diamagnetic, Ti 2+ like ions in inner layers and surface paramagnetic Ti + like centers with one unpaired electron per magnetic center. From a Heisenberg spin model, the relevant isotropic magnetic coupling constants are extracted from an appropriate mapping of total energy differences per formula unit to the expected energy values of the spin Hamiltonian. While the numerical values of the magnetic coupling constants largely depend on the used functional, the nearest neighbor intralayer coupling is found to be always ferromagnetic, and constitutes the dominant interaction, although two other non-negligible interlayer antiferromagnetic terms are involved, implying that the spin description cannot be reduced to NN interaction only. The influence of the MXene thickness is noticeable for the dominant ferromagnetic interaction, increasing its value with the MXene width. However, the interlayer interactions are essentially due to the covalency effects observed in all metallic solutions which, as expected, decay with distance. Within the PBE+ U approach, a U value of 5 eV is found to closely simulate the results from hybrid functionals for Ti 2 C and less accurately for Ti 3 C 2 and Ti 4 C 3 .

Fusing high-throughput quantum mechanical screening techniques with modern artificial intelligence strategies is among the most fundamental ─yet revolutionary─ science activities, capable of opening new horizons in catalyst discovery. Here, we apply this strategy to the process of finding appropriate key descriptors for CO 2 activation over two-dimensional transition metal (TM) carbides/nitrides (MXenes). Various machine learning (ML) models are developed to screen over 114 pure and defective MXenes, where the random forest regressor (RFR) ML scheme exhibits the best predictive performance for the CO 2 adsorption energy, with a mean absolute error ± standard deviation of 0.16 ± 0.01 and 0.42 ± 0.06 eV for training and test data sets, respectively. Feature importance analysis revealed d -band center (ε d ), surface metal electronegativity (χ M ), and valence electron number of metal atoms ( M V ) as key descriptors for CO 2 activation. These findings furnish a fundamental basis for designing novel MXene-based catalysts through the prediction of potential indicators for CO 2 activation and their posterior usage.

Platinum single-atom catalysts hold promise as a new frontier in heterogeneous electrocatalysis. However, the exact chemical nature of active Pt sites is highly elusive, arousing many hypotheses to compensate for the significant discrepancies between experiments and theories. Here, we identify the stabilization of low-coordinated Pt II species on carbon-based Pt single-atom catalysts, which have rarely been found as reaction intermediates of homogeneous Pt II catalysts but have often been proposed as catalytic sites for Pt single-atom catalysts from theory. Advanced online spectroscopic studies reveal multiple identities of Pt II moieties on the single-atom catalysts beyond ideally four-coordinated Pt II -N 4 . Notably, decreasing Pt content to 0.15 wt.% enables the differentiation of low-coordinated Pt II species from the four-coordinated ones, demonstrating their critical role in the chlorine evolution reaction. This study may afford general guidelines for achieving a high electrocatalytic performance of carbon-based single-atom catalysts based on other d 8 metal ions., (© 2023. The Author(s).)

This Perspective aims at providing a road map to computational heterogeneous photocatalysis highlighting the knowledge needed to boost the design of efficient photocatalysts. A plausible computational framework is suggested focusing on static and dynamic properties of the relevant excited states as well of the involved chemistry for the reactions of interest. This road map calls for explicitly exploring the nature of the charge carriers, the excited-state potential energy surface, and its time evolution. Excited-state descriptors are introduced to locate and characterize the electrons and holes generated upon excitation. Nonadiabatic molecular dynamics simulations are proposed as a convenient tool to describe the time evolution of the photogenerated species and their propagation through the crystalline structure of photoactive material, ultimately providing information about the charge carrier lifetime. Finally, it is claimed that a detailed understanding of the mechanisms of heterogeneously photocatalyzed reactions demands the analysis of the excited-state potential energy surface.

The nature of the electronic ground state of the Ti 2 C MXene is unambiguously determined by making use of density functional theory-based calculations including hybrid functionals together with a stringent computational setup providing numerically converged results up to 1 meV. All the explored density functionals (i.e., PBE, PBE0, and HSE06) consistently predict that the Ti 2 C MXene has a magnetic ground state corresponding to antiferromagnetic (AFM)-coupled ferromagnetic (FM) layers. A spin model, with one unpaired electron per Ti center, consistent with the nature of the chemical bond emerging from the calculations, is presented in which the relevant magnetic coupling constants are extracted from total energy differences of the involved magnetic solutions using an appropriate mapping approach. The use of different density functionals enables us to define a realistic range for the magnitude of each of the magnetic coupling constants. The intralayer FM interaction is the dominant term, but the other two AFM interlayer couplings are noticeable and cannot be neglected. Thus, the spin model cannot be reduced to include nearest-neighbor interactions only. The Néel temperature is roughly estimated to be in the 220 ± 30 K, suggesting that this material can be used in practical applications in spintronics and related fields., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

A high-throughput analysis based on density functional simulations underscores the viable epitaxial growth of MXenes by alternating nitrogen and metal adlayers. This is supported by an exhaustive analysis of a number of thermodynamic and kinetic thresholds belonging to different critical key steps in the course of the epitaxial growth. The results on 18 pristine N- and C-based MXenes with M 2 X stoichiometry reveal an easy initial N 2 fixation and dissociation, where N 2 adsorption is controlled by the MXene surface charge and metal d-band center and its dissociation controlled by the reaction energy change. Furthermore, formation energies indicate the plausible formation of N-terminated M 2 XN 2 MXenes. Moreover, the further covering with metal adlayers is found to be thermodynamically driven and stable, especially when using early transition metal atoms. The most restrictive analyzed criterion is the N 2 adsorption and dissociation at nearly full N-covered adlayers, which is yet achievable for almost half of the explored M 2 X seeds. The present results unfold the possibility of expanding, controlling, and tuning the composition, width, and structure of the MXene family.

Carbon interaction with transition metal (TM) surfaces is a relevant topic in heterogeneous catalysis, either for its poisoning capability, for the recently attributed promoter role when incorporated in the subsurface, or for the formation of early TM carbides, which are increasingly used in catalysis. Herein, we present a high-throughput systematic study, adjoining thermodynamic plus kinetic evidence obtained by extensive density functional calculations on surface models (324 diffusion barriers located on 81 TM surfaces in total), which provides a navigation map of these interactions in a holistic fashion. Correlation between previously proposed electronic descriptors and ad/absorption energies has been tested, with the d -band center being found the most suitable one, although machine learning protocols also underscore the importance of the surface energy and the site coordination number. Descriptors have also been tested for diffusion barriers, with ad/absorption energies and the difference in energy between minima being the most appropriate ones. Furthermore, multivariable, polynomial, and random forest regressions show that both thermodynamic and kinetic data are better described when using a combination of different descriptors. Therefore, looking for a single perfect descriptor may not be the best quest, while combining different ones may be a better path to follow., Competing Interests: The authors declare no competing financial interest., (© 2022 American Chemical Society.)

Transition metal carbides are increasingly used as catalysts for the transformation of CO 2 into useful chemicals. Recently, the effect of nanostructuring of such carbides has started to gain relevance in tailoring their catalytic capabilities. Catalytic materials based on molybdenum carbide nanoparticles (MoC y ) have shown a remarkable ability to bind CO 2 at room temperature and to hydrogenate it into oxygenates or light alkanes. However, the involved chemistry is largely unknown. In the present work, a systematic computational study is presented aiming to elucidate the chemistry behind the bonding of CO 2 with a representative set of MoC y nanoparticles of increasing size, including stoichiometric and non-stoichiometric cases. The obtained results provide clear trends to tune the catalytic activity of these systems and to move towards more efficient CO 2 transformation processes.

We present a computational study of the activity and selectivity of early transition-metal carbides as carbon dioxide reduction catalysts. We analyze the effects of the adsorption of CO 2 and H 2 on the (001), (011), and metal-terminated (111) surfaces of TiC and ZrC, as carbon dioxide undergoes either dissociation to CO or hydrogenation to COOH or HCOO. The relative stabilities of the three reduction intermediates and the activation energies for their formation allow the identification of favored pathways on each surface, which are examined as they lead to the release of CO, HCOOH, CH 3 OH, and CH 4 , thereby also characterizing the activity and selectivity of the two materials. Reaction energetics implicate HCO as the key common intermediate on all surfaces studied and rule out the release of formaldehyde. Surface hydroxylation is shown to be highly selective toward methane production as the formation of methanol is hindered on all surfaces by its barrierless conversion to CO., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Catalytic-materials design requires predictive modeling of the interaction between catalyst and reactants. This is challenging due to the complexity and diversity of structure-property relationships across the chemical space. Here, we report a strategy for a rational design of catalytic materials using the artificial intelligence approach (AI) subgroup discovery. We identify catalyst genes (features) that correlate with mechanisms that trigger, facilitate, or hinder the activation of carbon dioxide (CO 2 ) towards a chemical conversion. The AI model is trained on first-principles data for a broad family of oxides. We demonstrate that surfaces of experimentally identified good catalysts consistently exhibit combinations of genes resulting in a strong elongation of a C-O bond. The same combinations of genes also minimize the OCO-angle, the previously proposed indicator of activation, albeit under the constraint that the Sabatier principle is satisfied. Based on these findings, we propose a set of new promising catalyst materials for CO 2  conversion., (© 2022. The Author(s).)

Nanoscale titanium carbide (TiC) is widely used in composites and energy applications. In order to design and optimize these systems and to gain a fundamental understanding of these nanomaterials, it is important to understand the atomistic structure of nano-TiC. Cluster beam experiments have provided detailed infrared vibrational spectra of numerous Ti x C y nanoparticles with well defined masses. However, these spectra have yet to be convincingly assigned to Ti x C y nanoparticle structures. Herein, using accurate density functional theory based calculations, we perform a systematic survey of likely candidate nanoparticle structures with masses corresponding to those in experiment. We calculate harmonic infrared vibrational spectra for a range of nanoparticles up to 100 atoms in size, with a focus on systems based on removing either four carbon atoms or a single titanium atom from rocksalt-structured stoichiometric TiC nanoparticles. Our calculations clearly show that Ti-deficient nanoparticles are unlikely candidates to explain the experimental spectra as such structures are highly susceptible to C-C bonding, whose characteristic frequencies are not observed in experiment. However, our calculated infrared spectra for C-deficient nanoparticles have some matching features with the experimental spectra but tend to have more complex infrared spectra with more peaks than those obtained from experiment. We suggest that the discrepancy between experiment and theory may be largely due to thermally induced anharmonicities and broadening in the latter nanoparticles, which are not be accounted for in harmonic vibrational calculations.

Metal nanoparticles (NPs) are ubiquitous in many fields, from nanotechnology to heterogeneous catalysis, with properties differing from those of single-crystal surfaces and bulks. A key aspect is the size-dependent evolution of NP properties toward the bulk limit, including the adoption of different NP shapes, which may bias the NP stability based on the NP size. Herein, the stability of different Pd n NPs ( n = 10-1504 atoms) considering a myriad of shapes is investigated by first-principles energy optimisation, leading to the determination that icosahedron shapes are the most stable up to a size of ca. 4 nm. In NPs larger than that size, truncated octahedron shapes become more stable, yet a presence of larger {001} facets than the Wulff construction is forecasted due to their increased stability, compared with (001) single-crystal surfaces, and the lower stability of {111} facets, compared with (111) single-crystal surfaces. The NP cohesive energy breakdown in terms of coordination numbers is found to be an excellent quantitative tool of the stability assessment, with mean absolute errors of solely 0.01 eV·atom -1 , while a geometry breakdown allows only for a qualitative stability screening.

A density functional theory study is presented here aimed at investigating whether the atomic stacking on the new family of two-dimensional MXene materials has an influence on their adsorption properties and whether these properties can provide information about this structural feature. To this end, the Mo 2 C MXene, exhibiting two nearly degenerate crystal structures with either ABC or ABA atomic stacking, is chosen as a case study. The study of the adsorption of CO, CO 2 , and H 2 O on both polymorphs of Mo 2 C reveals substantial differences that could be used in experiments to provide information about the atomic stacking of a given sample. Particularly, we show that the asymmetric and symmetric stretching modes of the adsorbed CO 2 and the CO stretching mode are clear features that allow one to identify the stacking of atomic layers of the Mo 2 C MXene. The present finding is likely to apply to other MXenes as well., Competing Interests: The authors declare no competing financial interest., (© 2021 American Chemical Society.)

The interaction of CO 2 with nitride MXenes of different thickness is investigated using periodic density functional theory-based calculations and kinetic simulations carried out in the framework of transition state theory, the ultimate goal being predicting their possible use in Carbon Capture and Storage (CCS). We consider the basal (0001) surface plane of nitride MXenes with M n+1 N n (n=1-3; M=Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) stoichiometry and also compare to equivalent results for extended (001) and (111) surfaces of the bulk rock-salt transition metal nitride compounds. The present results show that the composition of MXenes has a marked influence on the CO 2 -philicity of these substrates, whereas the thickness effect is, in general, small, but not negligible. The largest exothermic activation is predicted for Ti-, Hf-, and Zr-derived MXenes, making them feasible substrates for CO 2 trapping. From an applied point of view, Cr-, Mo-, and W-derived MXenes are especially well suited for CCS as the interaction with CO 2 is strong enough but molecular dissociation is not favored. Newly developed kinetic phase diagrams are introduced supporting that Cr-, Mo-, and W-derived MXenes are appropriate CCS substrates as they are predicted to exhibit easy capture at mild conditions and easy release by heating below 500 K., (© 2021 The Authors. ChemPhysChem published by Wiley-VCH GmbH.)

The discovery of graphene has catalyzed the search for other 2D carbon allotropes, such as graphynes, graphdiynes, and 2D π-conjugated polymers, which have been theoretically predicted or experimentally synthesized during the past decade. These materials exhibit a conductive nature bound to their π-conjugated sp 2 electronic system. Some cases include sp -hybridized moieties in their nanostructure, such as acetylenes in graphynes; however, these act merely as electronic couplers between the conducting π - orbitals of sp 2 centers. Herein, via first-principles calculations and quantum transport simulations, we demonstrate the existence of an acetylene-meditated transport mechanism entirely hosted by sp -hybridized orbitals. For that we propose a series of nanostructured 2D materials featuring linear arrangements of closely packed acetylene units which function as sp -nanowires. Because of the very distinct nature of this unique transport mechanism, it appears to be highly complementary with π-conjugation, thus potentially becoming a key tool for future carbon nanoelectronics.

MXenes are two-dimensional nanomaterials isolated from MAX phases by selective extraction of the A component-a p -block element. The MAX exfoliation energy, E exf , is considered a chemical descriptor of the MXene synthesizability. Here, we show, by density functional theory (DFT) estimations of E exf values for 486 different MAX phases, that E exf decreases (i) when MAX is a nitride, (ii) when going along a metal M component d series, (iii) when going down a p -block A element group, and (iv) when having thicker MXenes. Furthermore, E exf is found to bias, even to govern, the surface chemical activity, evaluated here on the CO 2 adsorption strength, so that more unstable MXenes, displaying larger E exf values, display a stronger attachment of species upon.

Perdew-Burke-Ernzerhof (PBE) and PBE adapted for solids (PBEsol) are exchange-correlation (xc) functionals widely used in density functional theory simulations. Their differences are the exchange, μ, and correlation, β, coefficients, causing PBEsol to lose the Local Spin Density (LSD) response. Here, the μ/β two-dimensional (2D) accuracy landscape is analyzed between PBE and PBEsol xc functional limits for 27 transition metal (TM) bulks, as well as for 81 TM surfaces. Several properties are analyzed, including the shortest interatomic distances, cohesive energies, and bulk moduli for TM bulks, and surface relaxation degree, surface energies, and work functions for TM surfaces. The exploration, comparing the accuracy degree with respect experimental values, reveals that the found xc minimum, called VV, being a PBE variant, represents an improvement of 5% in mean absolute percentage error terms, whereas this improvement reaches ~11% for VVsol, a xc resulting from the restoration of LSD response in PBEsol, and so regarded as its variant., (© 2020 Wiley Periodicals LLC.)

Transition metal carbides have been long proposed as replacements for expensive Pt-group transition metals as heterogeneous catalysts for hydrogenation reactions, featuring similar or superior activities and selectivities. Combining experimental observations and theoretical calculations, we show that the hydrogenating capabilities of molybdenum carbide can be further improved by nanostructuring, as seen on MoC y nanoclusters anchored on an inert Au(111) support, revealing a more prominent role of Mo active sites in the easier H 2 adsorption, dissociation, H adatom diffusion, and elongated chemisorbed H 2 Kubas moieties formation when compared to the bulk δ-MoC(001) surface, thus explaining the observed stronger H 2 interaction and the larger formation of CH x species, making these systems ideal to catalyze hydrogenation reactions.

Due to their vast range of promising biomedical and electronic applications, there is a growing interest in bioinorganic lamellar nanomaterials. MXenes are one such class of materials, which stand out by virtue of their demonstrated biocompatibility, pharmacological applicability, energy storage performance, and feasibility as single-molecule sensors. Here, we report on first-principles predictions, based on density functional theory, of the binding energies and ground-state configurations of six selected amino acids (AAs) adsorbed on O-terminated two-dimensional titanium carbide, Ti 2 CO 2 . We find that most AAs (aspartic acid, cysteine, glycine, and phenylalanine) prefer to adsorb via their nitrogen atom, which forms a weak bond with a surface Ti atom, with bond lengths of around 2.35 Å. In contrast, histidine and serine tend to adsorb parallel to the MXene surface, with their α carbon about 3 Å away from it. In both adsorption configurations, the adsorption energies are on the order of the tenths of an electronvolt. In addition, we find a positive, nearly linear correlation between the binding energy of each studied AA and its van der Waals volume, which suggests an adsorption dominated by van der Waals forces. This relationship allowed us to predict the adsorption energies for all of the proteinogenic AAs on the same Ti 2 CO 2 MXene. Our analysis additionally shows that in the parallel adsorption mode there is a negligible transfer of charge density from the AA to the surface but noticeable in the N-bonded adsorption mode. In the latter, the isosurfaces of charge density differences show accumulation of shared electrons in the region between N and Ti, confirming the predicted N-Ti bond. The moderate adsorption energy values calculated, as well as the preservation of the integrity of both the AAs and the surface upon adsorption, reinforce the capability of Ti 2 CO 2 as a promising reusable biosensor for amino acids.

A density functional theory based study is presented with the aim of addressing the surface energy stabilization mechanisms of transition metal carbide and nitride surfaces from a crystal structure different from that of the most stable polymorph. To this end, we consider the MoC(001), MoN(001), WC(001), and WN(001) surface of rocksalt structures, which, for these compounds, is not the most stable one. The geometry optimization of suitable slab models shows that all these surfaces undergo a sensible reconstruction. The energy difference per formula unit between the rock salt and the most stable polymorph seems to be the driving force behind the observed reconstruction. A note of caution is given in that certain small periodic boundary conditions can artificially restrain such reconstructions, for which at least (2×2) supercells are needed. Also, it is shown that neglecting such a surface reconstruction can lead to artifacts in the prediction of the chemical activity and/or reactivity of these surfaces.

Cu@Pt nanoparticles (NPs) are experimentally regarded as improved catalysts for NO x storage/reduction, with higher activities and selectivities compared with pure Pt or Cu NPs, and with inverse Pt@Cu NPs. Here, a density functional theory-based study on such NP models with different sizes and shapes reveals that the observed enhanced stability of Cu@Pt compared with Pt@Cu NPs is due to energetic reasons. On both types of core@shell NPs, charge is transferred from Cu to Pt, strengthening the NP cohesion energy in Pt@Cu NPs, and spreading charge along the surface in Cu@Pt NPs. The negative surface Pt atoms in the latter diminish the NO bonding owing to an energetic rise of the Pt bands, as detected by the appliance of the d-band model, although other factors, such as atomic low coordination or the presence of an immediate subsurface Pt atom do as well. A charge density difference analysis discloses a donation/back-donation mechanism in the NO adsorption., (© 2020 Wiley-VCH GmbH.)